An electrolyzer is defined as an apparatus in which an electrolysis reaction takes place. Electrolysis is the process of decomposing a chemical compound into its elements or producing a new compound by the action of an electrical current. Basically, an electrolyzer is composed of two electrodes and a separator, called a membrane. Electrolyzers are commonly encountered in the chlor-alkali industry, in chlorate production plants, and in fuel cells. In the chlor-alkali industry, primary products of electrolysis are chlorine gas, hydrogen gas, and sodium hydroxide solution (commonly called “caustic soda” or simply “caustic”). Most electrolyzers are of the membrane cell type, but recently, new technologies such as G.D.E cells (Gas Diffusion Electrode) are being used in industrial settings for caustic soda and chlorine production. In the chlorate industry, sodium chlorate or sodium hypochlorite is produced from the electro-generated chlorine and caustic soda with no separator in the electrolysis cell. Analogously, in fuel cells, water is electrolyzed to produce hydrogen gas.
FIG. 1 is an illustration of a typical prior art membrane cell used in the chlor-alkali industry. It is composed of two compartments. The anode compartment is provided with a saturated brine solution (NaCl), while a dilute caustic soda passes through the cathode compartment. In chlor-alkali plants, chlorine gas (Cl2) evolves at the Titanium-coated anode 2. The combination of hydroxide ions (OH−) with migrated sodium ions(Na+) across the selective membrane 1 generates caustic soda (NaOH) and hydrogen gas (H2). The cathode 3 is usually nickel with a catalytic coating to reduce the over potential for H2 evolution. The complete chlor-alkali process is described by the following equation:2 NaCl+2 H2O→Cl2+H2+2 NaOH
Usually an electrolyzer is a combination of elementary membrane cells. Since the electrolysis process takes place in each cell after applying a current, energy consumption plays a key role in the process. The electrolyzer overall performance therefore is mainly related to cell efficiency. According to principles well known in the art, and described in literature such as “A First course in Electrode Processes” by Derek Pletcher, or in “Ion Permeable Membranes”, by Thomas A. Davis, J. David Genders and Derek Pletcher, voltage variations in the membrane cell are generally a result of physical changes within the cell components. The cell voltage variation is distributed between its components: anode, cathode, membrane and electrical connections. An abnormal decrease or increase in the cell voltage is generally considered as a source of potential problems.
It is therefore desirable to be able to monitor and characterize an electrolyzer's cell efficiency.
Commonly, when monitoring a process, measurement systems provide ways to define alarm thresholds for each of the monitored values. With this approach, it is often difficult to set the best threshold, since a fixed threshold does not take into account the changing context of the process. For instance, in an electrolyzer, the individual cell voltage will vary proportionally to the load of the system. At low load, reaching the threshold value will represent a much higher risk than the same level at a higher load.
Therefore, when setting a low level threshold with no external information, it is difficult to decipher if the anomaly arises at the process level or at the low production level.
Measurement sensors usually perform their readings assuming that their values are independent from the others. Often this is not the case. Several measurements depend on common process parameters.
Some approaches try to improve these shortcomings by linking measured signal values with external elements to combine the information and provide adjustments. Here again, there are some drawbacks since external values cannot be taken to the level at which efficiency is best. Usually, additional components or distant processes are needed to perform the adjustments which leads to deferring the action and more breaking modes are possible.
Additionally, it is sometimes difficult to synchronize signal values originating from different sources. Precision is lost which in turn affects the detection process.